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Receptor Kinase AXL is Modulated in the Osteogenic Differentiation of Human Mesenchymal Stromal Cells on Modified Titanium Implant Surfaces

Mohammad Ramine Khan, Nikolaos Donos, Vehid Salih and Peter Mark Brett

Titanium (Ti) implants with micro-rough topography and high surface free energy promote osseointegration, which in vitro analyses suggest is due to a novel enhancement in cellular osteogenic differentiation and function. The AXL receptor tyrosine kinase (AXL) is expressed on mesenchymal stromal cells (MSCs) and is implicated with its ligand, Growth arrest-specific 6 (GAS6), in the negative regulation of osteogenic differentiation, and may be modulated in the enhanced osteogenic differentiation of MSCs on modified Ti surfaces. This hypothesis was tested by culturing human MSCs on tissue culture plastic (TCP), polished (P), micro-rough-hydrophobic (SLA) and micro-rough hydrophilic (modSLA) Ti surfaces for seven days. Total RNA and protein levels of AXL and GAS6 were examined by real time PCR and ELISA, respectively. The effects of deregulating the signalling pathway in hMSCs with either receptor agonist or antagonist were investigated by analysing calcium mineralisation and soluble osteoblastic marker synthesis. The MSCs were found to significantly down-regulate AXL and GAS6 earlier on rough surfaces compared to smooth over seven days. Addition of the receptor agonist caused a relative decrease in calcium mineralisation that was most marked for TCP compared to any Ti surface. The antagonist did not affect mineralisation but caused a relative increase in osteoblastic soluble protein levels on rough surfaces only. Gene expression data showed an up-regulation of RUNX2 and beta-catenin with the receptor antagonist. These findings suggest that a down-regulation of AXL correlates with increased cellular mineralisation on the modified surfaces and that it might be a putative biomarker for assessing the clinical efficacy of endosseous implants.